1. Field of the Invention
The present invention relates to an image sensor used for facsimiles, scanners and the like, and more particularly to an image sensor having a wiring structure with a less electrical influence among interconnection wires.
2. Discussion of the Related Art
In the conventional image sensors, particularly close-contact type image sensors, there is known an image sensor of the type in which image data of a document is projected onto the sensor in a one-to-one correspondence and the projected image is converted into electrical signals. A TFT (Thin Film Transistor) driven image sensor belonging to the category of this type of the image sensor has been used. In the TFT driven image sensor, the projected image is divided into a great number of picture elements (photodetecting elements), charges generated in the photodetecting elements are temporarily stored every specific block of photodetecting elements in the wiring capacitances existing between the wires by using thin film transistor switching elements, and the charges are time-sequentially read out of the wiring capacitances in the form of electrical signals at a speed of several hundreds KHz to several MHz. In the TFT driven image sensor, the image can be read using a single drive IC, through the operation of the TFT. Therefore, the number of drive ICs for driving the image sensor can be reduced.
The TFT driven image sensor, as shown in FIG. 8 showing an equivalent circuit of the image sensor, is made up of a linear photodetecting element array 51 having a length substantially equal to the width of a document, a charge transfer section 52 consisting of a plurality of thin film transistors Ti,j (i=1 to N, j=1 to n) provided in association with photodetecting elements 51" in one-to-one correspondence, and a matrix-arrayed, multilayered wiring structure 53.
The photodetecting element array 51 is divided into photodetecting element groups of an "N" number of blocks. The "n" number of photodetecting elements 51" forming one group may be equivalently expressed by photo diodes Pi,j (i=1 to N, j =1 to n). The photodetecting elements 51" are respectively connected to the drain electrodes of the thin film transistors Ti,j.
The source electrodes of the thin film transistors Ti,j are respectively connected, every group of photodetecting elements, to the "n" number of common signal lines 54 through the multilayered wiring structure 53. The signal lines 54 are connected to a drive IC 55.
The gate electrodes of the thin film transistors Ti,j are connected to a gate pulse generator 56 so that the transistors of each block are rendered conductive simultaneously. The optical charges generated in the photodetecting elements 51" are stored, for a predetermined period of time, in the stray capacitance of each photodetecting element and the overlap capacitance between the drain and gate of each thin film transistor. Then, the charges are sequentially transferred in each block to the wiring capacitances Ci (i=1 to n) of the multilayered wiring structure 53. During the charge transfer operation, the thin film transistors Ti,j serve as charge transfer switches.
A gate pulse .phi.G1 is transferred from the gate pulse generator 56 through the gate signal lines G1 to the thin film transistors T1,1 to T1,n of the first block, to turn them on. The charges generated in the photodetecting elements 51" of the first block are transferred to and stored in the wiring capacitances Ci. By the charges stored in the wiring capacitances Ci, the potentials in the common signal lines 54 is varied. The varied voltages are time-sequentially introduced onto an output line 57 by successively turning on analog switches SWi (i=1 to n) in the drive IC 55.
In response to the gate pulses .phi.G2 to .phi.Gn, the transistors T2,1-T2,n to TN,1-TN,n in the second to N-th blocks are turned on, so that the charges of the photodetecting elements are transferred every block, thereby obtaining an image signal of one line in the first scan direction on an original document. The original is moved by means of a document feed means (not shown), such as rollers, and the sequence of the operations as stated above is repeated. Finally, image signals of the whole document are obtained (Japanese Patent Application Unexamined Publication No. Sho. 63-9358).
FIG. 9 is a plan view showing the construction of the multilayered wiring structure, and FIG. 10 is a cross section taken on line C--C' in FIG. 9. As shown in these figures, the multilayered wiring structure 53 is constructed with a substrate 21, lower layer signal lines 31, an insulating layer 33, and an upper layer signal lines 32. In the structure, the signal line 31, the insulating layer 33 and the signal lines 32 are multilayered on the substrate 21. The signal lines 31 and 32 are arranged crossing each other. Contact holes 34 are provided for interconnection of the upper and lower signal lines.
As described, in the construction of the conventional image sensor, the multilayered wiring structure has the matrix construction such that the upper and lower signal lines cross each other with the insulating layer 33 being interlayered therebetween, as shown in FIG. 10 Accordingly, a coupling capacitance exits at each cross point of the lower and upper layer signal lines 31 and 32. The coupling capacitance causes a potential difference between the upper and the lower lines at each cross portion. The output signal from one of the signal lines is influenced by the output signal from the other. That is, crosstalk occurs preventing the charge from being detected exactly. Hence, the tone or gradation reproduction of the image sensor is deteriorated.
To cope with the problem, there is proposed an image sensor having a photodetecting element array including a plurality of blocks linearly arrayed in the first scan direction, each block consisting of a preset number of photodetecting elements, a plurality of switching elements for transferring charges generated in the photodetecting elements of each block, and a drive IC for outputting the charges in the form of image signals, wherein the switching elements in a block of the photodetecting element array and the switching elements in another block located adjacent to the former block are connected by wires in such a way that the switching elements closest to each other between the blocks are interconnected, the switching elements next close to each other are interconnected, and so on, and the wires connecting the switching elements in a block to the switching elements in blocks on both sides of the former block are disposed oppositely with respect to the first scan direction, and are disposed in such a way that the shortest wire connecting them is located closest to the photodetecting element array, the next shortest wire is located next closest to the photodetecting element array, and so on.
The image sensor thus arranged is free from the interference among the signal lines, and ensures an exact read of charges from the wiring capacitances because the signal lines do not cross each other.
However, in the image sensor, an "n" number of signal lines in parallel to each other meander through the photodetecting element array, and coupling capacitances are present among those parallel signal lines. Therefore, a potential difference is present between the adjacent signal lines, so that one signal line is influenced by the other adjacent to the former, and crosstalk occurs. As a result, an exact potential detection is impossible, deteriorating the tone reproduction performance of the image sensor.
When load capacitors are formed in the wiring portion of the image sensor, the load capacitors for the signal lines must be uniform in capacitance in order to exactly read the charges from the signal lines. When downsizing of the image sensor is intended, the areas for the load capacitors must be decreased.
In the image sensor, the signal lines within the wiring structure are electrically influenced from one another since the potentials in the signal lines vary owing to the charge transfer. In this case, the signal line located farthest from the photodetecting element array is electrically influenced by the signal line located on the inner side of the former, but is not influenced from its outside because no signal line is disposed outside the outermost signal line. The electrical influence received by the outermost signal line is different from that of the inner signal lines. Therefore, the output voltages of the signal lines are not equal.